Electromagnetic Pulse (EMP)

An electromagnetic pulse is a transient electromagnetic
signal produced by a nuclear explosion in or above the Earth's atmosphere.
Though not considered (directly) dangerous to people, the electromagnetic
pulse (EMP) is a potential threat to many electronic signals.

In a typical nuclear detonation, parts of the shell casing and other
materials are rapidly reduced to a very hot, compressed gas, which upon
expansion gives rise to enormous amounts of mechanical and thermal energy.
At the same time the nuclear reactions release a tremendous amount of
energy as initial nuclear radiation (INR). This INR is in the form of
rapidly-moving neutrons and high-energy electromagnetic radiation, called
x-rays and prompt gamma rays. Roughly a minute after detonation, the radioactive
decay of the fission products gives rise to additional gamma rays and electrons (or beta particles), known as residual nuclear radiation (RNR).
The distribution of the total explosive energy of a hypothetical fission
detonation in the atmosphere below an altitude of 6 miles (10 km) is 50%
blast (mechanical), 35% thermal, 10% RNR, 5% INR. At higher altitudes
where the air is less dense, the thermal energy increases and the blast
(mechanical) energy decreases proportionally.

EMP is associated with the INR output, which is a small percentage of
the total explosive energy. Nevertheless, EMP is still capable of transferring
something of the order of 0.1 - 0.9 joule/m2 (0.007 - 0.06
ft-lbf/ft2) onto a collector, more than enough to cause upset
or damage to normal semiconductor devices.

As prompt gamma rays move away from a high-altitude nuclear detonation,
those gamma rays moving toward the Earth penetrate a more-dense region
of the atmosphere called the source or depletion region. In this 6-mile
(10-km) region, approximately 15 - 21 miles (25 - 35 km) above the Earth,
the highly-energetic gamma rays interact with the air molecules to form
Compton electrons (with energies starting at 1 MeV) and less-energetic
gamma rays, which then proceed in the same general direction as the original
gamma rays. The fast Compton electrons eventually slow down by stripping
other electrons from the air molecules to form secondary electron-ion
pairs. (Though these secondary electrons and ions don't contribute to
the generation of the EMP, they do cause the region to become highly conductive, and therefore play an important role in determining the EMP wave shape and amplitude.) While slowing down, the very-intense, short-duration flux
of Compton electrons is also deflected by the Earth's geomagnetic field.
The Compton electrons then spiral about the geomagnetic lines, radiating
electromagnetic energy in the form of EMP until they eventually recombine
with local, positively-charged ions

It's also possible for INR (both x-rays and gamma rays) to directly
interact with systems, causing EMP signals internal to structures. This
phenomenon has been called internal or system-generated EMP and is potentially
a serious problem for satellites in orbit or for electronics in metallic
enclosures on or near the ground. These forms of EMP are generated by
x-rays interacting with satellites and gamma rays impinging on ground-based
enclosures, producing currents of Compton electrons internally that then
produce electromagnetic waves.

An estimate of about 1 joule (0.7 ft-lbf) of EMP-coupled energy is considered
reasonable for many systems. Even if the coupling onto circuits is inefficient,
as little as 10-13 J can upset some semiconductor devices and 10-6 J can cause damage. The potential for such upset and damage
in critical electronic circuits has led to the incorporation of EMP protection
in many system designs. The protection is most prevalent in communication
systems whose disruption by EMP is considered an important civil and military
vulnerability.

The most common form of protection incorporated in system designs is
a combination of shielding and penetration control. The diagram below
shows a protection scheme in which a system's electronics E is isolated
from the external environment by one or more nested, shielded enclosures
(often called Faraday cages). Penetration control is then maintained by
minimizing the number of shield penetrations (in this case, a power line,
a signal line, and a ground wire connecting E to earth ground G) and by
applying terminal-protection devices, such as spark gaps, Zener diodes,
or metal-oxide varistors, at selected shield-penetration points (A, A',
A", B, B', and B"; or Z, Z', and Z"; or both). In this
way, system protection can be designed not only for EMP but also for other
electromagnetic transients (such as near-strike lightning and electromagnetic
interference). Furthermore, cost-effective, field-maintainable protection
can be achieved properly selecting off-the-shelf shielding techniques and terminal-protection devices and applying them to the systems.